1. Field of the Invention
The present invention relates to an industrial fabric such as a papermaker's fabric, a fabric for producing nonwoven fabric, a fabric used for the removal or squeezing of water from sludge or the like, a belt for producing a constructional material or conveyor belt and, particularly, to a papermaker's fabric, more particularly, to a papermaker's forming fabric.
2. Description of Related Art
Conventionally used industrial fabrics include papermaker's fabrics, such as papermaker's forming fabrics and papermaker's canvasses, fabrics for producing nonwoven fabric, fabrics for removing water from sludge and the like, belts for producing construction materials, conveyor belts, and many others. Dimensional stability is required for these industrial fabrics to prevent elongation or shrinkage in a width direction because when in use, the fabrics are running while they receive tensile force in a warp direction. Running stability and attitude stability (or non-changeability in shape) are also required for these fabrics to prevent zigzag running or wrinkling.
Abrasion resistance is further required because the fabrics contact a driving roll or the like and can be worn away while they are running. Further, as they carry or process an object installed on their surfaces, their surfaces must be smooth.
The above problems are common to industrial fabrics but are yet to be solved.
Papermaker's fabrics are often the most necessarily required to have these properties, as compared to other industrial fabrics. Particularly, papermaker's forming fabrics must have properties for paper making which will be described hereinafter, in addition to the above properties. When a papermaker's forming fabric is described, most problems which are common to industrial fabrics and solutions to these can be described and understood. Therefore, the present invention will be described hereinafter, using a papermaker's forming fabric as a typical example. However, the present invention relates to any type of fabric, and is not limited to papermaker's forming fabric.
A paper making method is a known technology, in which a paper making raw material including pulp fibers or the like is first supplied onto a running papermaker's forming fabric which is formed endless from a head box and laid between rolls of a paper making machine.
A side to which the raw material is supplied to the papermaker's forming fabric is known as a paper making surface and the opposite side is known as a running surface.
The supplied raw material is transferred along with the running of the papermaker's forming fabric, and water is removed from the raw material by a dehydrator such as a suction box or foil installed on the running side of the fabric while it is transferred, thereby forming a wet web. That is, the papermaker's forming fabric functions as a type of filter and separates pulp fibers from water.
The wet web formed in this paper making zone is transferred to a press zone and a drier zone. In the press zone, the wet web is transferred to a papermaker's felt and then carried so that water is squeezed out from the wet web at a nip pressure between press rolls together with the papermaker's felt and further removed. In the drier zone, the wet web is transferred to a papermaker's canvass, carried and dried to make paper.
The papermaker's fabric is woven of warps and wefts such as synthetic resin monofilament yarn by a loom. An endless fabric is formed by known seaming, pin seaming or the like or with a hollow weaving machine in a weaving stage.
In the case of hollow weave, the relationship between warps and wefts is reversed in the loom and at the time of use.
In this specification, the term "warp" means a yarn extending in the mechanical direction of a paper making machine, that is, a running direction of a fabric and the term "weft" mans a yarn extending in the crosswise direction of the paper making machine, that is, a width direction of the fabric.
There are many requirements for a papermaker's fabric, particularly a papermaker's forming fabric. The requirements include the improvement of surface smoothness, the prevention of the formation of wire marks on paper, the improvement of retention, high water filtration properties, abrasion resistance, dimensional stability, running stability, and the like.
In recent years, solutions to the above requirements have been strongly desired, along with desires to increase paper making speed, the paper making of neutralized paper and the consumption of a filler, and a desire to follow a cost reduction policy adopted by paper making companies.
When the paper making speed is increased, the dehydration speed is inevitably accelerated and dehydration force becomes powerful. Since a raw material for paper making is dehydrated through a papermaker's forming fabric, water is removed through meshes formed between the yarns of the papermaker's forming fabric. This mesh space is water filtration space. However, since not only water but also fine fibers, a filler and the like are removed from the raw material for paper making, the yield of produced paper is lower. As the wet web formed on the fabric is pressed against the paper making surface of the fabric by dehydration force, a yarn bites into the wet web in a portion where it is existent and conversely, the wet web bites into space between meshes where no yarn is existent, whereby there is a strong tendency toward the formation of yarn and mesh mark on the surface of the wet web.
Since the density of fibers is excessively increased by the long residence of fibers between meshes, the density of fibers on the paper becomes uneven and the thickness of paper becomes nonuniform. This is called "wire mark" or "water filtration mark".
If the bite of the fabric into the wet web is large or the sticking of fibers occurs, the releasability of the wet web deteriorates when the wet web is transferred to the felt. Although it is impossible to eliminate the wire marks completely, the paper making surface of the fabric must be made fine and fiber supporting properties and smoothness must be improved to minimize them and prevent them from standing out.
If the dehydration speed is high and the dehydration force is powerful, the removal of fibers and the formation of wire marks become marked, thereby making it necessary to further improve the above properties.
Since fibers are aligned in a running direction of the fabric, the fiber supporting properties of wefts in particular must be improved.
Excellent water filtration properties are required to remove water efficiently at a high speed. If the water filtration properties are excellent, it is possible to reduce the vacuum pressure of dehydration, suppress the above-described bite of fibers into space between meshes and the removal of the fibers, eliminate the formation of wire marks, and improve the yield.
If the paper making speed is high, water contained in the fabric is scattered by the centrifugal force of the rotation unit of a roll or the like to form sprays of water which fall on the wet web to form marks. Therefore, the water retention properties of the fabric must be reduced.
Meanwhile, the requirement for the improvement of abrasion resistance is made stronger by the growth of paper making of neutralized paper. Since calcium carbonate is used as a filler in the paper making of neutralized paper, it greatly wears away a yarn on the running surface, unlike clay used in acidic paper making. Further, excessive water filtration caused by an increase in paper making speed or a reduction in water filtration due to the residence of fibers makes conditions more severe.
To improve abrasion resistance, the structure of the fabric is made a weft abrasion type structure, or the material of yarn is changed.
Generally speaking, with a view to improving the abrasion resistance and maintaining the attitude stability of a fabric in use, it is preferred to provide the wefts of the fabric with an abrasion resisting function. If warps wear away, the fabric stretches and wears away due to a reduction in its tensile strength as a matter of course. If the warps further wear away and break, the fabric itself breaks and its service life ends. Therefore, the abrasion of the warps is prevented by the wefts.
An attempt has been made to use polyamide monofilament yarn having excellent abrasion resistance as a weft. However, this attempt does not improve the structure of the obtained fabric itself but makes use of the properties of a material used. Therefore, a remarkable effect cannot be obtained and there is such a defect that the attitude stability of the fabric is poor because the polyamide monofilament has small rigidity.
An attempt has also been made to use thick yarn as a weft on the running surface. However, this involves such a defect that the balance between warps and wefts is lost with the result of deterioration in crimping properties and the formation of wire marks and has a problem to be solved for practical application.
To prevent the formation of wire marks on paper, it is conceivable to increase the numbers of warps and wefts so as to improve fiber supporting properties. To this end, the line diameters of a warp and a weft must be reduced.
However, a known dual layer fabric having an upper layer of weft yarns and a lower layer of weft yarns weaving with a single layer of warp yarns, which is generally used now, deteriorates in abrasion resistance, rigidity and attitude stability when the line diameters of a warp and a weft are reduced.
In this way, when the line diameters are increased to improve abrasion resistance and rigidity, the surface properties of a papermaker's fabric are impaired and wire marks are formed on paper. On the other hand, when the line diameters are reduced and the numbers of warps and wefts are increased to improve surface properties, abrasion resistance and rigidity deteriorate. Therefore, the above properties conflict with one another.
The above abrasion resistance and attitude stability problems are common to all industrial fabrics which have no ends and rotate.
To solve the above problems, an attempt has been made to produce a fabric formed by using different warps and wefts for the paper making side and the running side thereof and integrating both layer fabrics with binder yarn. That is, warps and wefts having small line diameters are used to form a fine paper making surface of a fabric on the paper making side, and warps and wefts having large line diameters are used to form a running surface having large abrasion resistance of a fabric on the running side.
However, this has not always been satisfactory because, in a connection portion where a binder yarn and a yarn on the paper making side cross each other, a depression is formed on the surface of the fabric on the paper making side as the binder yarn pulls the fabric on the paper making side toward the running side and the depression mark is transferred to paper is made actually, thereby forming a wire mark.
When the line diameter of the binder yarn is reduced or the number of the binder yarn is reduced to eliminate the depression as much as possible, the connection force is weakened, whereby the binder yarn is wrinkled between the fabric on the paper making side and the fabric on the running side, thereby causing internal friction. As a result, the binder yarn breaks or stretches and further connecting force is weakened, whereby a gap is formed between the fabric on the paper making side and the fabric on the running side, these fabrics are separated from each other, and hence, the service life of the obtained fabric ends in a short period of time.
To improve fiber supporting properties efficiently and make high-quality paper without forming wire marks on paper, pulp fibers must be suitably supported by wefts. This is because the pulp fibers which are supplied onto the papermaker's forming fabric from the head box are generally aligned in a mechanical direction, that is, a warp direction. It is possible to prevent fibers from staying between warps by dividing a depression between warps by wefts and supporting fibers.
However, this does not mean that the paper making surface may be formed with wefts alone. A fabric must have a portion where a warp is located above a weft and the warp and the weft form the same plane, thereby making it possible to form a smooth paper making surface having no wire mark. It is necessary to improve the fiber supporting properties of wefts while the same plane is formed.
The requirement for the improvement of rigidity, particularly rigidity is a width direction, is becoming important as the paper making speed increases, a tendency toward instantaneous dehydration becomes more marked, and conditions for papermaker's forming fabrics become more severe year after year.
When rigidity in the width direction is low, a wavy wrinkle is formed during running and paper gathers more in a depression portion of the wrinkle than in a projection portion, paper of the depression portion becomes thick and heavy as a matter of course, and paper of the projection portion becomes thin and light, thus producing unevenness in weight in the width direction, that is, a so-called BD failure.